Mobile stations are also known as e.g. mobile terminals, wireless terminals and/or user equipments (UE). Mobile stations are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system. The communication may be performed e.g. between two mobile stations, between a mobile station and a regular telephone and/or between a mobile station and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Mobile stations may further be referred to as mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The mobile stations in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the mobile stations within range of the base stations.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for mobile stations. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
According to 3GPP/GERAN, a mobile station has a multi-slot class, which determines the maximum transfer rate in the uplink and downlink direction. GERAN is an abbreviation for GSM EDGE Radio Access Network. EDGE is further an abbreviation for Enhanced Data rates for GSM Evolution.
In the context of this disclosure, the expression DownLink (DL) is used for the transmission path from the base station to the mobile station. The expression UpLink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
More and more communication is related to communication in a mixed cellular and Device-to-Device (D2D) communications environment.
Cell Search and Cell Selection in LTE
Before a user equipment can transmit or receive information to/from an 3GPP LTE network, it must search and select an appropriate serving cell, derive basic system information and perform random access. These steps together are often referred to as “initial access” and enable a user equipment to obtain synchronization, fundamental system parameters and to get access to system resources.
The first step, i.e. cell search and selection, is necessary for the user equipment to synchronize both in time and frequency to the so called Primary Synchronization Signals (PSS) and Secondary Synchronization Signals (SSS) that together encode the different LTE cells' Physical (PHY) layer cell identity, the so called PHY Cell ID or “PCI” or “RD” for short, composed by the physical layer cell identity group (0 . . . 167) and the physical layer identity (0, 1 or 2). The PSS and SSS signals are constructed in a special way that makes it possible for an LTE user equipment after power up, i.e. increasing its power, to find and lock onto these signals and decode the PCI.
Once the user equipment has successfully performed cell search and cell selection, it is able to decode the PHY Broadcast CHannel (PBCH) and read out the so called Master Information Block (MIB). The MIB comprises essential system information, such as the system bandwidth and the number of transmit antennas at the base station side that the user equipment needs to know for being able to communicate with the LTE network.
Once the user equipment has decoded the MIB it has enough information to decode the System Information Blocks (SIBs) transmitted on the DL Shared CHannel (DL-SCH). There are multiple SIBs ranging from 1 and upwards. Out of these SIBs, SIB1 and SIB2 are essential for accessing the network and begin UL transmission. SIB1 and 2 comprises information about the configuration of the different radio channels in the cell, e.g. PHY UL Control CHannel (PUCCH), Paging Control Channel (PCCH) and Random Access CHannel (RACH), as well as cell barring information indicating which user equipments are allowed to access the cell and whether a user equipment should refrain from camping on a cell even if it provides coverage at the location of the user equipment.
Finally, the Random Access (RA) procedure is used to obtain, as well as to re-establish, uplink synchronization and an initial grant to uplink transmission resources. During RA, the LTE eNB sends a RA Response to the user equipment that comprises for example time alignment instruction, as well as initial PHY Uplink Shared CHannel (PUSCH) resource grant that together allow the user equipment to transmit uplink data and/or signaling. Once the user equipment has performed the RA procedure the user equipment and eNB are able to communicate with each other controlled by dedicated signaling over the DL-SCH.
Device-to-Device (D2D) Communications in LTE
Device-to-device (D2D) communication has been promoted as a means to provide peer-to-peer services between user equipments, facilitate infrastructure-less communications in emergency, and National Security and Public Safety (NSPS) situations and to enhance network capacity by offloading traffic from the radio access network.
A design option for implementing D2D communications between LTE user equipments is to adopt a Bluetooth master-slave concept such that one of the communicating user equipments takes the master role and closely emulates an LTE micro eNB. In such a design the Master user equipment provides the synchronization signals PSS and SSS, as well as the essential system information, e.g. MIB and some of the SIBs such as SIB1 and 2. It also implements the standard RA procedure allowing for a similar initial access for the Slave user equipment as a regular cellular eNB.
Although the eNB emulation makes the Master user equipment appear as a “real” eNB, there are also some significant differences that are not transparent to the Slave user equipment. For example, the services provided by a Master user equipment are largely different since the Master user equipment may not even be connected to a mobile core network.
Home Base Stations and Femtocells in LTE
The increasing demand for indoor service coverage has triggered the standardization of so called femto cells and home base stations both by the 3GPP such as Home Node B (HNB) for UMTS and Home eNodeB (HeNB) for LTE, and by the Institute of Electrical and Electronics Engineers (IEEE) communities. A femtocell is served by a small cellular base station, typically designed for use in a home or small business. Although a HeNB provides similar basic functionalities towards an LTE user equipment, a HeNB differs from a macro eNB in terms of output transmit power, mobility support, access control such as Closed Subscriber Groups (CSG) handling, operation and maintenance procedures and possibly also in terms of the provided services. Note that handover to/from a macro eNB may not be transparent.
Location Registration and Paging in LTE
A new principle has been introduced for location registration in System Architecture Evolution (SAE)/LTE networks. This principle is based on a Tracking Area (TA) concept in a similar way as Location Areas (LA) and Routing Areas (RA) in GSM and WCDMA networks. Each SAE/LTE cell belongs normally to a single TA, if not considering RAN sharing deployments, and the Tracking Area Identity (TAI) is broadcasted as part of System Information (SI). The TAI comprises a Mobile Country Code (MCC), a Mobile Network Code (MNC) and a Tracking Area Code (TAC).
The main difference from the LA/RA concepts is that in SAE/LTE a concept called multiple TAs and associated TAI List has been introduced. This means that the network may return a TAI List to the user equipment as part of some EPS Mobility Management (EMM) procedures like Attach, Tracking Area Update (TAU) and Globally Unique Temporary Identity (GUTI) Reallocation. As long as the user equipment camps on a cell belonging to a TA who's TAI is included in the current TAI List, the user equipment does not perform normal TAUs (periodic TAUs are still performed). The user equipment performs normal TAU first when it moves to a cell that does not belong to a TA in the TAI List. As part of this TAU, the user equipment receives a new TAI List and the same procedure continues.
As the network knows the user equipment location on the TAI List level, this means that the Paging Area is also normally all the TAs included in the TAI List. The Paging procedure is used to inform a user equipment in Radio Resource Control (RRC)-IDLE state about an “incoming call” and the need for the user equipment to move to the RRC-CONNECTED state. When the user equipment is in a RRC-IDLE state it is in a state where no connection to a base station is established yet. When the user equipment is in a RRC-CONNECTED state it is in a state wherein a connection to a base station is established. Paging may also be used for other purposes like to inform the user equipment about System Information change or about ETWS (Earthquake and Tsunami Warning System) related notifications.
The Paging Channel (PCH) provides also support for user equipment discontinuous reception (DRX) to enable user equipment power saving. This is achieved by the user equipment in idle mode monitoring the PDCCH channel for a P-RNTI (Paging Radio Network Temporary Identifier) value indicating paging. The user equipment only needs to monitor the PDCCH channel at certain user equipment-specific occurrences i.e. specific subframes within specific radio frames, see below. At other times, the user equipment may apply DRX by for example switching off the receiver to preserve battery power.
In a mixed radio environment, an LTE user equipment may need to be able to communicate via the different types of access points and with different other user equipments. One example is the case when a user equipment is active in a D2D communication with one or more other user equipments and when any of these user equipments would like to be able to receive paging in the “macro coverage” i.e. from an LTE eNodeB simultaneously with the D2D communication.
Today methods require that the devices engaged in a D2D session have the same paging occasions. Typically, user equipments being served in a cell may have paging occasions at separate time instances which may be determined prior to the devices starting a D2D session. Thus, in today's methods, the user equipment that is engaged in simultaneous D2D and cellular communication may miss cellular paging. This applies mainly for single-receiver user equipments, i.e. cases when the same physical radio equipment is used for both macro and local communications, or in scenarios where dual receiver is not feasible, e.g. the carriers used in the macro and local communication are too close.